List-Post: [email protected] Date: Tue, 29 Jul 1997 17:09:52 -0400 (EDT) Joe,
>I have a question that I hope some of you can help me with. >I have heard many stories about business travelers who have >destroyed their PC card modems by plugging them into digital >PBXs. >1) How widespread is this problem? I don't know, partly because my information is "dated." About seven years ago I looked into this (when I worked at GTE's quality control laboratory), and it was pretty common at the time. I have heard (but I don't know this for sure) that many of the modern modems, especially PC Card (PCMCIA) are being designed to avoid this problem. I'm not sure that it is easily done though, because the easiest way to design a modem to meet the "power cross" requirements tested for in UL 1459 is to put a fuse in it. If this fuse is a low enough value to work for the UL 1459 tests, it will definitely blow on a "digital PBX power cross." (After all, a power cross is a power cross, no matter where it comes from!) >2) Which digital PBX vendors use a connection scheme that can >cause this problem? Most of them now. In the early days of digital PBXs, many were "hybrid," and used two or even three twisted pairs per phone, and the power was typically "simplexed" on multiple pairs, so it didn't hurt an analog device that was accidently plugged in. Nowadays, most PBX digital phones work on a single pair, and the phone power is typically on that pair (rather than use an AC wall pack for each phone). I know for a fact that the single-pair digital phone lines for the entire GTE OMNI PBX product line (now obsolete, but plenty of them are still in the field) will cause problems with analog modems due to current surges, and I suspect that most other brands of PBX will as well. >3) What are the electrical characteristics of the voltage placed >on tip/ring (open circuit voltage and short circuit current)? >With regard to question #3 above, I would expect that the open >circuit voltage is either 24 volts or 48 volts. These voltages >are well within the range that an analog modem can handle, but >perhaps the source resistance is so low that large currents end >up flowing through the modem DAA. I would like to know how large >these potential currents are. The voltage is usually 24 VDC (but might be 48 VDC) -- the problem is that the short-circuit current is much higher than you would see from a PSTN line -- in the case of the old GTE digital PBX lines it was about 350 mA (a PSTN line would be 80-120 mA maximum, depending on the telco and/or country). BTW, IBM makes a "modem saver device" which is about the size of a thick ball-point pen -- you plug it into a RJ-11 jack at the hotel, and LEDs on the device let you know whether the line is safe for your analog modem! (I don't recall the name of the device, but other people sell it as well, with their own name attached.) ________________________________________________________________ John Combs, Senior Project Engineer, ITS/TestMark Laboratories Email: [email protected] URL: http://www.testmark.com
